1. Field of the Invention
The present invention relates to a magnetic random access memory using the spin injection technique, a data read method of the same, and a resistance random access memory.
2. Description of the Related Art
While the degree of integration of NAND flash memories is steadily increasing with time, needs for nonvolatile RAMs (Random Access Memories) are still high. Examples of candidates for nonvolatile RAMs presently being developed are an FeRAM (Ferroelectric Random Access Memory) and MRAM (Magnetic Random Access Memory).
Recently, an MRAM using spin injection magnetization reversal as a write method is being developed (e.g., non-patent reference 1). In 1996, Slonczewski and Berger proposed a spin injection technique of reversing magnetization by the direct interaction between conduction electron spins and the magnetic moment by applying a polarized spin electric current to a magnetic cell. It was confirmed that a GMR (Giant Magneto-Resistive) element and TMR (Tunneling Magneto-Resistive) element operated by this technique.
The spin injection technique as described above eliminates the problem of disturbance in a half-selected state, which arises when each cell is made up of one memory cell and one selection element and data is written by using a biaxial current magnetic field. The spin injection technique thus makes selective data write feasible.
Unfortunately, an MRAM using the spin injection technique poses the following problem in a read operation.
When reading out data, a read current is applied to a cell to read out the change in electric current or voltage caused by the resistance state of a memory element, in the same manner as in an ordinary MRAM. That is, the operation of applying an electric current to a memory element is performed in both data read and write. When reading out data, therefore, the problem (read disturbance) that data is written in a memory element by a read current may arise. When using a method that forms reference signals for data read from dummy cells in which data “0” and “1” are prewritten, the probability of read disturbance occurring in these dummy cells is highest.
In the prior art, a read current I is supplied in the same direction (from a free layer f to a pinned layer p) in both magneto-resistive elements of two reference cells RC in which data “0” and “1” are written. Therefore, the read current I is supplied in the same direction as a write current in one reference cell RC, and supplied in the direction opposite to the write current in the other reference cell RC. Accordingly, the probability of read disturbance occurring in the latter reference cell RC rises because the frequency at which the read current I flows through the reference cell RC is higher than that of an ordinary memory cell MC. This problem is serious from the viewpoint of quality control since an MRAM having no sequence of rewriting data in the reference cell RC is regarded as a defective product.
Non-patent reference 1: 2005 SYMPOSIUM ON VLSI TECHNOLOGY, p. 184
Patent reference 1: Jpn. Pat. Appln. KOKAI Publication No. 2004-220759 (e.g., FIG. 16)